1. Field of the Invention
The present invention relates to a substrate that is used for a liquid crystal display device, a liquid crystal display device having the substrate and a method of manufacturing the liquid crystal display device, and more particularly to a substrate that maintains a uniform cell gap in spite of self-weight, a liquid crystal display device having no unfilled-region, and a manufacturing the liquid crystal display device preventing the overflow of the liquid crystal molecules.
2. Description of the Related Art
Generally, a liquid crystal display device includes a thin film transistor substrate, a color filter substrate and a liquid crystal layer interposed between the thin film transistor and the color filter substrate.
When a thickness of the liquid crystal layer (or a cell gap between the thin film transistor substrate and the color filter substrate) is non-uniform, a display quality of the liquid crystal display device is deteriorated.
A spacer is interposed between the thin film transistor substrate and the color filter substrate, so as to maintain the cell gap between the thin film transistor substrate and the color filter substrate.
The spacer has a spherical shape or a column-shape. The spacer that has a spherical shape is referred to as a ball spacer. The spacer that has a column-shape is referred to as a rigid spacer.
A diameter of the ball spacer is only a few μm. The ball spacer is scattered on the color filter substrate or on the thin film transistor substrate.
The ball spacer has demerits as follows.
Firstly, the ball spacer is deformable and scattered irregularly, so that the cell gap is not uniform. Secondly, a liquid crystal molecule near the ball spacer is abnormally arranged to lower a luminance. Thirdly, reducing the diameter of the ball spacer is very hard, so that making a short cell gap is also very hard. Fourthly, the ball spacer may be disposed on a pixel to deteriorate a display quality.
A photoresist formed on the thin film transistor substrate or on the color filter substrate is etched to form the rigid substrate. Thus, the rigid substrate may be formed in a region between the pixels, so that the display quality is not deteriorated. Further, the photoresist may have a thin thickness. Thus, the cell gap may be reduced.
However, when a different compression is applied depending on regions, the cell gap is different according to the regions.
For example, when the thin film transistor substrate is over the color filter substrate with reference to a gravitational force, the thin film transistor substrate sags due to an atmospheric pressure and a self-weight of the thin film transistor substrate.
FIG. 1 is a graph showing a relation between a cell gap and a position of a thin film transistor substrate, when a general rigid spacer is used.
A point ‘A’ represents a position near a first edge of a thin film transistor substrate. A point ‘B’ represents a center position of the thin film transistor substrate. A point ‘C’ represents a position near a second edge that is opposite to the first edge of the thin film transistor substrate.
Referring to FIG. 1, a cell gap at the center position ‘B’ of the thin film transistor is minimum, and the cell gap increases in a direction from the center position ‘B’ to the edge position ‘C’. That is, because compression is maximum at the center due to a self-weight of the thin film transistor substrate.
Thus, a rigid spacer formed at the center ‘B’ is compressed more than a rigid spacer formed at the edges ‘A’ and ‘C’, so that the rigid spacer formed at the center ‘B’ or the center portion of the color filter substrate and the thin film transistor substrate may be damaged.
Recently, as a liquid crystal display apparatus becomes larger, a method of filling the liquid crystal is changed from a vacuum injection method into a drop and filling method.
In the drop and filling method, liquid crystal material is dropped on the color filter substrate having a spacer formed thereon. Then, the color filter substrate and the thin film transistor substrate are assembled with each other.
The drop and filling method is simple in comparison with the vacuum injection method. However, a liquid crystal display apparatus manufactured by the drop and filling method may include an unfilled region where the liquid crystal is not completely filled, or the liquid crystal material may overflow, when the liquid crystal material is too much.
In detail, when the liquid crystal material is provided too much, the liquid crystal material overflows, so that the thin film transistor substrate and the color filter substrate are not completely assembled. When the liquid crystal material is provided insufficiently, the liquid crystal display apparatus includes the unfilled region.
The unfilled region deteriorates a display quality of the liquid crystal display apparatus.
Due to these problems, a vacuum injection method is used in spite of complexity of procedure and much consumption of liquid crystal material.
However, as the liquid crystal display apparatus becomes larger, the vacuum injection method meets a limit. Thus, the drop and filling method has been developed.
In the drop and filling method, the amount of the spacer and the liquid crystal material is important.